Asphalt is a solid or semisolid cementitious material having as its main constituents bitumens which occur in nature or are obtained by refining petroleum. For years, asphalt coating compositions applied in the manufacture of roofing materials have contained mineral filler or stabilizer. This filler has been in powder form and has been included in the range of from 0 to 70 percent by weight of the total composition. Typical filler materials are: limestone, dolomite, nepheline syenite, or ground shale.
One type of mineral sold as a particulate filler is a rock filler having approximately the following composition:
______________________________________ Weight Percent ______________________________________ SiO.sub.2 50-75 Al.sub.2 O.sub.3 14-20 Fe.sub.2 O.sub.3 3-l0 CaO 1.5-12 MgO 0.5-8 Na.sub.2 O 2-7 K.sub.2 O 0.3-7 ______________________________________
This particulate is characterized by the following properties:
______________________________________ Particle size - &lt;250 micrometers (.mu.) Mean particle size - 40-150.mu. Hardness - 5-7 Mohs Specific gravity - 2.5-3.1 ______________________________________
Although it is in good supply, the above-described particulate is not considered as good a filler as limestone because it does not wet-out as well in asphalt as limestone and cannot be mixed at as high a loading as limestone (e.g., 65-70 weight percent). A greater concentration of filler allows less of the much more expensive asphalt to be incorporated into the total composition.
The term wet-out means the affinity or compatibility of the filler for the asphalt. Poor wet-out is indicated by the following conditions occuring when the filler and asphalt are mixed: filler staying on top of the mixture; or filler agglomeration. Good wet-out is indicated by rapid wetting of particles by the fluid asphalt. A qualitative indication can be given by observing the rate at which filler is wetted out by molten asphalt during mixing.
Roofing granules having the same composition given above (which go on top of shingles) have been coated with oil to control dust.
There is a need for a treatment for the above-described filler particles to enhance their compatibility and thus their usefulness as asphalt filler. If the treatment could be economically applied and if the treated particles could be dispersed into asphalt with the same ease as limestone particles, they would be an economically attractive alternative to limestone.